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Weird battery readings on MP 790

PDXMark Page Icon Posted 2012-10-16 6:07 PM
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I just bought a used MobilePro 790, and I'm not sure if the battery is bad or there's some other issue. It seemed to hold a charge the first couple days. Then I mistakenly put a battery from a 770 in it, thinking it was a 790 battery. I took it out later that day and put the 790 battery back in, but since that time the battery readings have been all over the place. When I have it on the adapter, it charges to 60% and no further. As soon as I unplug the adapter it drops to 40%, and then quickly down to 20%. It then fluctuates between 20 and 10%. It gives frequent low battery warnings and shuts itself down, but it hasn't actually run out of juice.

So I'm wondering if this is a problem with the battery or with the software.....both Resource Meter and WR-Tools give the same readings. Would putting the wrong battery in cause problems? Or does this sound like just a bad battery?
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Rich Hawley Page Icon Posted 2012-10-16 7:01 PM
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Sounds like the battery is failing to me. Putting in the wrong battery should make absolutely no difference. What is the rating on the AC adapter...is it a standard NEC adapter or some lower rated adapter? More than once I've seen sellers who will charge a battery on a good machine and then sell it with the device knowing that it's not going to charge properly with what they sent.
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PDXMark Page Icon Posted 2012-10-16 7:11 PM
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The adapter is the standard NEC MC-BA6 - I had it from a previous 790. This unit came w/o an adapter. The battery is the NEC MC/BA14A, 7.4 V, 1700mAh.

One other detail: when I plug in the adapter, the charging light turns green immediately, indicating a full charge.

Edited by PDXMark 2012-10-16 7:33 PM
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Rich Hawley Page Icon Posted 2012-10-16 8:17 PM
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That's almost always a sign that the battery is shot. Sometimes when people have been charging that battery with an AC adapter of a lower capacity, then it doesn't fully charge it, the battery loses its ability to fully charge, kind of resetting itself to a lower capacity level. The lower amperage of the charger actually resets the battery. Let it totally deplete and then see what happens, see if you can then recharge it fully again. Sometimes you can force charge the thing back to where it should be...

BTW, if it is bad, don't throw it out. As long as the termistor is good in it, it should be fine for rebuilding...
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PDXMark Page Icon Posted 2012-10-16 10:33 PM
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I'll try draining it, Rich, and see if that shocks it back to normal. I had wondered if that might work.

You've posted instructions for re-building the battery somewhere, right? I'll have to take a look and see how much work it would be.
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Rich Hawley Page Icon Posted 2012-10-17 12:53 PM
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neill Page Icon Posted 2012-10-17 6:22 PM
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The MP battery looks very very similar to the battery in (my) a Jornada 728. My battery went dead a few months ago. Getting it open was a PITA till I read somewhere that the easiest way to open them is to twist each end till they pop open. I tried, with trepidation, but it did pop open easily, and two new 18650 cells later I was back in business.
BTW, I didn't even solder the new cells in place. Just made sure the wires that connect the terminals were in decent contact, they're held in place as the plastic shell's a tight fit. So far, 8 months later, still no problems.
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Rich Hawley Page Icon Posted 2012-10-17 7:59 PM
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If you make good contact and everything is tight, then no problems. I often use conductive glue that seems to work well as an alternative to soldering the contacts onto the lion cells.
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PDXMark Page Icon Posted 2012-10-18 2:01 AM
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Is there a quick way to drain the battery? I've got several programs running on it, but it won't run out of juice. Part of the problem is that it keeps shutting itself down because the battery reading is so low. But I don't think it really is that low, because it just won't run out of juice. I keep turning it back on, it stays on for a bit (sometimes a few seconds, sometimes a minute or two), it turns itself off, and we repeat. Is there a better way to do this?
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C:Amie Page Icon Posted 2012-10-18 10:30 AM
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If you have some wire you could try and discharge it into something that doesn't have any transistors, say a 8/9v motor. If you aren't comfortable with knowing where the terminals are and how to do it without blowing something up DON'T DO IT.
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